This is a sort of sister thread for my look at Vancian magic from last week. Looking at what you actually do in D&D (generally, not specifically the modern take), this is what I get:

Plenty of freeform negotiation of situations (which I’ve sort of already dealt with last year in my discussion of challenge-based adventuring); despite some weak efforts to the contrary, the core D&D experience really runs on the basis of you-imagine, I-imagine, the result of which is a set of mutually accepted (credible, in theory-speak) challenge constraints that are then set in stone until the challenge is completed.

The magic system, which is the most important resource subsystem in the game. Increasingly so at higher levels, increasingly so in later editions.

The combat system, which is what you do with that positioning and those resources.